One method for purifyinq gas streams, such as natural gas, is by washing with a solvent that absorbs the impure components (primarily acid gases such as carbon dioxide and hydrogen sulfide). The solvent is continuously circulated through a wash column or absorber where the solvent is loaded with acid gases. After the rich solvent is reduced in pressure, heated, and steam stripped in a regenerator column to remove the acid gases, the lean solvent is cooled and returned to the absorber.
The mechanism of absorption involves the release of heat due to the exothermic heats of absorption and reaction. Heat is consumed in the regenerator as the absorption process is reversed through desorption or stripping.
The use of aqueous mono- and diethanolamine solutions as solvents in these absorption/regenerator systems is well known as is the problem of mono and diethanolamine degradation. Degradation of mono- and diethanolamines is believed to be caused (i) by the presence of certain impurities such as hydrogen sulfide, carbonyl sulfide, and mercaptans in the gas stream to be purified and (ii) by air in those processes where air is allowed to contact the gas stream. Unfortunately, degraded mono- or diethanolamines have to be replaced frequently with the attendant cost of fresh monoor diethanolamine solutions and the time lost by the shutdown of the absorption/regeneration process.
To combat degradation, trialkanolamines, such as triethanolamine, triisopropanolamine, and triisobutanolamine, are introduced into the mono- or diethanolamine solutions. See U.S. Pat. No. 3,535,260, which is incorporated by reference herein. The trialkanolamines, however, suffer from the disadvantage of being much higher boiling and having a higher molecular weight than mono- or diethanolamine thus they cannot be recovered during the regeneration step.